1. Field of the Invention
The present invention relates to an optical pickup, an optical disc device, a polarization adjustment plate, and an optical device, which are desirably applied to an optical disc device that uses a laser diode as a light source, for example.
2. Description of the Related Art
In an optical disc device, by irradiating a laser beam from a laser diode to an optical disc, information is reproduced from or recorded to the optical disc.
As shown in FIG. 1, the light intensity of a laser beam 2 irradiated from a laser diode substantially expresses the Gaussian distribution, and only the central part of the laser beam 2 whose light intensity is large is made to go into the optical path in an optical pickup to be used as an optical beam. The light intensity distribution along a direction parallel with the active layer of the laser diode (referred to as θ// direction, hereinafter) is largely different from that along a direction perpendicular to the active layer (referred to as θ⊥ direction, hereinafter).
In case of directly making the optical beam go into the optical path in the optical pickup, as shown in FIG. 2B, with respect to the θ⊥ direction of the optical beam, since the optical disc device can make only part of the optical beam whose intensity is large and intensity distribution is flat go to an objective lens and set up an incidence region IEθ⊥, a light intensity distribution in which the ratio of the rim intensity RM indicative of the light intensity of the rim part of the objective lens with respect to the central light intensity Ps of the optical beam (referred to as rim intensity ratio, hereinafter) is large to some extent can be obtained, which can reduce the spot size.
On the other hand, as shown in FIG. 2A and FIG. 3A, with respect to the θ// direction of the optical beam, since the optical disc device makes also part of the optical beam whose intensity is small and intensity distribution is steep go to an objective lens and sets up an incidence region Ieθ//, the rim intensity RM of the optical beam becomes extremely small, and accordingly the rim intensity ratio becomes small, which undesirably enlarge the spot size of the optical beam to be irradiated to an optical disc.
As shown in FIG. 3B, in the optical disc device, by setting the distance from the laser diode 1 to a collimator lens 25 long, since only the central part of the laser beam 2 can be made to go to an objective lens 27 after widely diffusing the laser beam 2, the rim intensity ratio in the θ// direction can be made large. On the other hand, of the laser beam 2, part which is not made to go to the objective lens 27 is increased, which undesirably reduces the light use efficiency.
There is known an optical disc device in which a wavelength plate is divided into three regions, and part of an optical beam transmitting the central part thereof is converted from the P-polarization to the S-polarization, while both end parts of the optical beam transmitting both the end regions thereof are made to transmit in the intact state of the P-polarization, and part of the central part of the optical beam of the elliptical polarization is reflected by a polarization beam splitter. Accordingly, only the light amount of the central part of the optical beam transmitting the polarization beam splitter is reduced (for example, refer to Patent Document 1: Jpn. Pat. Appln. Laid-Open Publication No. 2006-185474).